One of the known post-translational modifications of proteins is the oxidation of methionine residues to methionine sulfoxide [Met(O)]. An enzyme, peptide methionine sulfoxide reductase (MsrA), which has been detected in virtually all organisms examined, catalyzes the reduction of free and protein-bound methionine sulfoxide residues to methionine. Peptide-methionine sulfoxide reductases (MsrAs) from different organisms share a consensus amino acid sequence (GCFWG) that is believed to play an important role in their catalytic activity. Site- directed mutagenesis of these amino acids in yeast MsrA resulted in a total loss of enzymic activity only when each CFW sequence was mutated. Nevertheless, all of the recombinant MsrA mutants and native proteins have a very similar circular dichroism (CD) spectrum. The enzyme was inactivated by iodoacetamide, which is known to bind to thiol groups. This observation is consistent with the notion that the cysteine in the consensus motif is essential for enzymic activity, since replacement of this cysteine with serine leads to total inactivation of the enzyme. The recombinant yeast MsrA yields a Km value for methionine sulfoxide similar to that of bovine MsrA and three times lower than the Km of the bacterial enzyme. We also found that the enzymic activity increases dramatically with increasing ionic strength. The recombinant yeast MsrA exhibits an absolute stereospecificity for the reduction of the d- sulfoxide diastereomer, independent of free or protein-bound methionine sulfoxide. To our surprise, while studying the physiological effects of this stereospecificity, we found a methionine auxotroph yeast strain that can grow on either diastereomer of L-methionine sulfoxide. Currently, the physiological roles of the MsrA are being studied under normal and oxidative stress conditions using msrA gene knock-out mice. - oxidative stress, methionine sulfoxide, T-cells, yeast, free radicals